The present invention relates in general to detecting misfires occurring during normal operation of an internal combustion engine and, more specifically, to an improved signal processing system and method for detecting misfires in a reciprocating engine.
Present engine control systems often include misfire detection systems. As emission standards become increasingly stringent, there is a need for accurate misfire detection and reporting under all engine operating conditions.
Monitoring crankshaft engine angular velocity and crankshaft acceleration are preferred techniques for detecting misfires of individual cylinder firings during engine operation. Both of these measurements depend largely upon engine torque produced during the combustion process to determine misfiring of particular engine cylinders. Given the velocity or acceleration information, misfires are predicted by various signature analysis and/or spectral analysis methods.
Several present misfire detection systems are well suited for detecting misfires in low data rate applications. Low data rate systems typically seek to detect misfires based on entire cylinder event intervals as measured by, for example, the profile ignition pulse (PIP) signal. In a four cylinder engine, the PIP signal actually indicates the approach to top dead center of two engine cylinders, one of which is approaching a power stroke and one of which is approaching an intake stroke. This is so because it takes two full crankshaft rotations to complete an engine cycle. In engines having more than four cylinders, however, the power strokes of different cylinders will overlap. Accordingly, associating crankshaft acceleration fluctuations with any particular cylinder becomes more difficult because the firing induced crankshaft accelerations occur over smaller rotational angles.
Accordingly, in engines having more than four cylinders, it is desirable to monitor average crankshaft acceleration over smaller crankshaft rotational intervals for improved misfire detection. As a practical matter, however, the engine angular velocity and acceleration behavior is also affected by powertrain-related behaviors other than firing torque. These other conditions can significantly reduce the signal-noise ratio of the firing torque-related velocity or acceleration signal under analysis. In addition, under certain engine operating conditions, the noise exceeds the engine torque-related velocity or acceleration signal under analysis. Moreover, noise is introduced into the crankshaft acceleration signal due to crankshaft torsional vibrations, inertial torque due to reciprocating masses, and other mechanically induced vibrations on the engine's crankshaft. This noise can hide or mask any signatory behavior of a misfire event.
One example of a high data rate misfire detection system with improved signal fidelity is disclosed in U.S. Pat. No. 5,515,720. In that patent, the improved fidelity acceleration signal is provided by a median filter operating on an acceleration signal where the median filter's rank is programmable dependent upon the determined operating condition of the engine.
The data stream relied upon by such high data rate systems, however, comprise interlaced non-homogeneous data points. Accordingly, crankshaft acceleration signatures based upon such data can mask or hide misfire events. In other words, when data is sampled at higher rates, i.e., at smaller crankshaft rotational angles, the samples are interlaced and represent different portions of the engine cycle. When these interlaced non-homogeneous samples are coupled with heuristic-type filters, such as a median filter which is designed to determine some root nature of a signal or the difference between the measured value and the root nature of a signal, errors are often introduced. In addition, if the signal is modified prior to being operated upon by these rule-based filters, the root nature of the signal is also modified.
Accordingly, there is a need for an improved misfire detection system and method which accounts for the small non-homogeneous angular sampling period of high data rate systems and the complications associated with interlaced data in heuristic-type filters in determining deviant accelerations or velocities to identify loss of combustion quality in internal combustion engines.